1. Field of the Invention
This invention relates to an organic electroluminescent device (OLED) that includes an emission layer composed of a phosphorescent dopant. The organic electroluminescent device has a low driving voltage, an excellent current density, a long lifetime, and high luminous efficiency.
2. Discussion of the Background
Phosphorescent materials are organic metal compounds having heavy atoms. Phosphorescent materials emit light when the relaxation of triplet excitons occurs because the relaxation of triplet excitons is a forbidden transition. Fluorescent materials use singlet excitons having a generation probability of 25%. Phosphorescent materials use triplet excitons having a generation probability of 75%, and therefore have better luminous efficiency than fluorescent materials.
An emission layer composed of a phosphorescent material may include a host and a dopant. The dopant receives energy from the host, thus emitting light. Dopants have been continually developed. Princeton University and South California University have presented materials using an Ir metal compound, for example.
Recently, an emission layer composed of a carbazole compound as a host has been developed. The carbazole compound has a larger triplet-state energy band gap than 4,4′-biscarbazolylbiphenyl (CBP). However, the efficiency and lifetime of phosphorescent devices that use previously known carbazole compounds are far below the desirable level.